1. Field of the Invention
The present invention relates to a data transmitting-receiving system.
2. Description of the Prior Art
Various systems are used for transmitting and receiving data. Whichever system is employed, attention must be paid to the reliability of the data.
A known data transmitting-receiving system includes a transmitter unit and a receiver unit connected together via a transmission line.
The transmitter unit includes a first level converter which produces an analog level signal according to a combination of the first digital input data of a plurality of channels allotted to a first channel route, and a first mixer receiving, at its first input, a first analog output from the first level converter and, at its second input, a first carrier signal. The transmitter unit also includes a second level converter producing analog level signal according to a combination of the second digital input data of a plurality of channels allotted to a second channel route, a second mixer receiving, at its first input, a second analog output from the second level converter and, at its second input, a second carrier signal and a hybrid circuit for adding the analog outputs of the first and second mixers with each other. An amplifier for amplifying the output from the hybrid circuit to a transmission power level is also provided.
The receiver unit includes a first demodulator and a second demodulator, which reproduce transmission signals corresponding to the first and second channel routes, respectively. The first and second demodulators then demodulate the thus reproduced analog level signals into first digital output data and second digital output data, respectively.
In the above prior art system, various factors cause data error. The factors are, for example, non-linearity of the amplifier in the transmitter unit, modulation distortion of the mixer in the transmitter unit, temperature variation, and aging of the system itself. Many proposals have been made for coping with these problems individually to improve data reliability, but few proposals have been made for coping with all of these problems at once.